The present invention is generally related to a method and machine used for the mass analysis of known and unknown chemical compounds. More specifically, the current invention is related to method of obtaining analytical mass spectrum data that is free from computational errors for a chemical compound and corresponding machine for identifying an unknown compound from experimental mass spectrum data using simulating mass spectrum data.
Mass spectrometry. Generally, mass spectrometer is a weighing machine for molecules. The basic principle of a mass spectrometer is that accelerated ions of atoms can be deflected into a detector by magnetic fields. Many patents have been issued describing different mass spectrometers, (e.g. U.S. Pat. No. 2,769,910; U.S. Pat. No. 2,818,507; U.S. Pat. No. 2,939,952; U.S. Pat. No. 2,950,389; U.S. Pat. No. 3,334,225; and U.S. Pat. No. 5,089,702 which are hereby incorporated by reference). Mass spectrometry is a technique for measuring the mass/charge ratio of molecular ions, and over the last 100 years many methods of creating molecular ions have evolved, including: an Electrospray (“ESI”) ion source; an Atmospheric Pressure Chemical Ionization (“APCI”) ion source; an Atmospheric Pressure Photo Ionization (“APPI”) ion source; a Matrix Assisted Laser Desorption Ionization (“MALDI”) ion source; a Laser Desorption Ionization (“LDI”) ion source; an Inductively Coupled Plasma (“ICP”) ion source; an Electron Impact (“EI”) ion source; a Chemical Ionization (“CI”) ion source; a Fast Atom Bombardment (“FAB”) ion source; and a Liquid Secondary Ions Mass Spectrometry (“LSIMS”) ion source).
In practice, a chemical sample is ionized, accelerated, deflected, and then the mass and charge of the ion is detected. However, ionization typically leads to further fragmentation of the test compound. The masses of these molecular fragments are also measured, which gives insight to the specific class of molecule that is being examined. The analysis of mass spectrometry data can be simplified further by spreading out the timing of the arrival of the individual component ions or fragmentation ions of a chemical mixture to the mass spectrometry detector. For this reason, many mass spectrometers are used in conjunction with other analysis tools such as gas chromatography and liquid chromatography. Reducing the number of different molecular species in the mass spectrometer at any one time simplifies the separation of mass spectrum peaks. This procedure works for chemical samples that contain on the order of 10 to 20 different molecular ion species, but may be inadequate for analyzing samples that contain thousands of different species. Mass spectrometry is a widely used technique for the identification of molecules, both organic and inorganic chemistry. Examples of useful mass spectrometry analysis include drug development, drug manufacture, pollution control analysis, and chemical quality control.
By understanding how specific fragments of an original compound break up during ionization, it is possible to generate a chemical structure of an original molecule from the mass spectrum data of that compound. Similarly, it should be possible to generate a simulated mass spectrum data for a known chemical structure. Currently, there is not a big interest in mass spectra simulations because many investigators are usually only interested in obtaining the mass of a specific species from mass spectra. Simulated mass spectrum data can be utilized as a reference for comparing experimental mass spectrum data for an unknown compound to determine the unknown compound's chemical structure. However, as with all scientific techniques, the devil is in the details.
Fourier Transforms and Simulated Mass Spectra. Many mass spectrometers include software that generate simulated mass spectra that can be used for comparing experimentally generated mass spectra to aid a user in the determination of the identity of a chemical compound. Unfortunately, the “state of the art” mass spectra simulations often give results that are not an exact match, and at best are interpreted as: “. . . it looks like . . . ,” which is not a very scientific interpretation of experimental data for an unknown compound. Additionally, published results sometimes are erroneous, which is a deterrent in using simulations of mass spectra as an analysis tool.
Generally, mass spectrum simulation software performs Fourier Transforms on subject compounds in order to generate the simulated mass spectra data that is used for comparison with experimental results. Because the Fourier Transform is an approximation, calculation errors are inherent and there is no control over the errors involved in calculation. For example, one such Fourier Transform method is disclosed in an article by Alan Rockwood and Steven Van Ordern titled “Ultra High-Speed Calculation of Isotope Distribution” in Analytical Chemistry Volume 68, No. 13, dated Jul. 1, 1996, (“the Rockwook '1996 Paper”), and is hereby incorporated by reference. Briefly, the Rockwood '1996 Paper describes a method based on (1) temporarily setting the masses of the isotopes to their nucleon numbers rather than their true masses, (2) calculating the mass distribution of the compound using a Fourier Transform-based method that produces correct intensity ratios for the nominal isotope peaks, and (3) adjusting the mass scale to correct for the errors made in the mass scale by step (1).
In general, isotope distribution calculation techniques utilize polynomial expressions generated by a formula (a+b)n, for an element with two stable isotopes, where “a” represents the relative abundance of the light isotope, “b” represents the relative abundance of the heavy isotope, and “n” represents the number of atoms of the particular element in the particular compound. As is apparent, a large number of terms will be present as the number of atoms, i.e., “n” increases. Thus, there remains a need for a system and method capable of quickly determining the identity of a subject compound without the accompanying error provided by prior art systems.
The actual mass of a molecule and the actual mass of its fragmentation ions are useful pieces of information for the identification of an unknown molecule, or in the identification of a known molecule in an unknown mixture of molecules. Mass spectrometers are extremely sensitive and even small changes in isotopes abundance of a compound can be detected, which complicates an experimental mass spectrum of an unknown compound. In contrast, simulated mass spectrum data contain inherent calculation errors that generally approximate isotope abundances. Such approximations, in turn, produce unreliable results when comparing directly the simulated spectrum data with the experimental mass spectrum data. The invention disclosed herein describes a method of generating simulated mass spectrum data that is free from calculation errors, which allows a user to obtain the theoretically correct mass spectrum of a chemical species of interest and compare the experimental mass spectrum data of an unknown compound with a database of or simulated mass spectra. The invention described herein is an improvement over Fourier Transform-based methods of determining the mass spectrum data of a chemical structure.